A device is described in German Patent No. DE 101 39 595 in which the state of motion of the preceding vehicle, that is, the vehicle that is located immediately in front of the host vehicle, is detected with the aid of a radar sensor. If both the preceding vehicle and the host vehicle are stationary and the host vehicle remains stationary longer than a predefined time span, the engine is automatically switched off. If it is then established with the aid of the radar sensor that the preceding vehicle has been set in motion again, the engine is automatically restarted. This device thus makes it possible to save a considerable amount of fuel at a high level of comfort to the driver in stop and go situations in a traffic jam, for example. In this instance, the decision regarding the switching on and off of the engine does not need to be exclusively a function of the state of motion of the preceding vehicle, but may additionally be a function of other parameters, for example, the state of motion of other motor vehicles that are located in adjacent lanes (vehicles driving away in the adjacent lane generally indicate that a traffic jam is clearing up) or also the state of a traffic signal (traffic light) that is detected with the aid of a video system.
The engine of the motor vehicle is usually an internal combustion engine for which the starting operation requires a certain amount of time. Since with the aid of the known device the starting operation is initiated only when a movement of the preceding vehicle is detected, undesired delays may occur when a traffic jam is clearing up. On the other hand, there may also be situations in which, from the standpoint of saving fuel, it is unfavorable to automatically switch off the engine. For example, this is the case if the engine must already be restarted after a standstill of less than approximately three seconds. This case also results in an undesired load on the engine starter and the vehicle battery.